This application pertains to the art of resistance welding and more particularly to a method and apparatus for automatically controlling resistance welding equipment.
The invention is particularly applicable to robot or other production line welding equipment which automatically makes a large number of consecutive welds without direct operator intervention or servicing. The plurality of welds may be made by performing the same welding function on each of a plurality of workpieces moving along an assembly line, by performing a number of different welding functions on each of a plurality of workpieces moving along an assembly line and the like. It will be appreciated, however, that the invention may find application in many types and styles of resistance welding equipment, including relatively simple and manually operated welders.
In resistance welding, a pair of electrodes are clamped on either side of the metallic workpieces which are to be welded and a welding current is then applied across the electrodes. As the current flows through the workpieces, the resistance of the metal and the interface conditions cause heating which is sufficient to melt the metal. It has been found that the resistance across the metal stock or workpieces during welding varies according to a predictable pattern. Although the resistance may be erratic during the first few milliseconds that the welding current is applied, the general trend is for the resistance to increase as the stock is heated. At some point during the weld, the resistance peaks and then begins to decrease. It is theorized that the resistance fluctation during the first few cycles is attributable to interface conditions, e.g., oil films, oxidation, surface conditions, the mating relationship or fit between the surfaces and the like. As further theorized, the increase in resistance is attributable to the higher resistance of hot and molten metals. Also, the decrease in the weld resistance after the peak is attributable to metal fusion and the resulting destruction of the interface resistance of the parts being welded. It has been found that the nugget size of the weld is roughly related to the percentage drop in the weld resistance from its peak value to the termination of the weld.
It has further been found that the quality of a weld is related to the weld time or duration. Particularly, it has been found that for given metallic stock to be welded, there is an optimum weld duration. If the predetermined percentage drop in resistance occurs in less than the optimal time or requires longer than the optimal time to occur, the weld is likely to be weak. As the weld duration becomes more divergent from the optimal weld time, the chances of the weld being bad or weak become greater. Thus, it is advantageous to set a low time limit to the weld to which the weld is continued regardless of the percent drop in resistance. Similarly, it is advantageous to set a high time limit to each weld at which the weld is terminated regardless of the drop in resistance.
It has been found that the repeated clamping of the weld electrodes over a large number of welds tends to cause the electrodes to mushroom, i.e., expand in cross-sectional area. Applying the same weld current through mushroomed electrodes decreases the current density, i.e., the weld current per unit area of electrode cross section. The decreasing weld current density from the mushrooming is apt to increase weld time sufficiently so that it exceeds the high limit on substantially all welds. Also, variations in the stock during a production run can cause the weld time to be beyond or outside the high or low limit.
Heretofore, welding controls have determined whether the weld time is between the high or low limits. The commonly assigned U.S. Pat. No. 3,588,438 issued June 28, 1971 to Peter W. Vanderheist shows a welding control that sounds an alarm when the weld is not between the high and low limits. When the alarm sounds, the welding machine operator manually makes appropriate adjustments to bring the weld time back into the optimal range. Such adjustments include the percent heat, reshaping or adjusting the cross-sectional area of the electrodes and the like. Others have previously suggested that a compensation can be made for electrode mushrooming by incrementing the percent heat by a small amount each time the welder completes a preselected number of welds.
One problem with prior art welding controls has been a lack of accuracy in adjusting the percent heat as the electrodes mushroom or the stock itself varies. There is a tendency for the operator to over or under adjust the percent heat in response to a high or low limit alarm. Frequently, the percent heat is alternately increased and decreased in an attempt to zero in on the continuously changing optimum percent heat.
The present invention contemplates a new and improved method and apparatus for controlling welding equipment which overcomes all of the above-referenced problems and others. The invention also provides a method and apparatus for controlling the percent heat which is very reliable and highly accurate.